Conditional access systems for broadcast applications are used for example in pay television operations wherein one has to subscribe to be entitled to watch a channel, a service or an event. In view of the high numbers of subscribers and thereby high numbers of secure devices distributed among the subscribers, the secure devices are open to attack by unauthorised persons trying to extract the unique key of a secure device. The secure devices are generally provided in the form of smart cards. If an unauthorised person or pirate succeeds to extract the unique key, the smart card is cracked and the pirate is able to distribute global keys over the internet, for example the first key or service key, to set up a pirate subscriber network.
When the service provider establishes that the conditional access system has been hacked, it is necessary to trace the cracked smart card or smart cards. A known method to trace a cracked smart card is to perform a so-called binary search on the base of smart cards which have been issued. According to this known method, a false key is published to half the smart cards and a true key to the other half. The pirate will publish either the false or true key on the internet and from the key published by the pirate, it is possible to narrow down to which half of the base the cracked smart card belongs. In a next step again a false key is published to one half of this located half of the base of smart cards and a true key to the other half of the base and again it can be deduced from the key published by the pirate in which half the cracked smart card is located. By repeating these steps, the cracked smart card can be located and switched off. A disadvantage of such a binary search is that legal subscribers will continuously see blacked out screens during the time the search is performed. If such search activities are repeatedly necessary and continue for long periods, this will lead to subscriber dissatisfaction.